Week 10.10.2022 – 16.10.2022

Quantum field theory of higher-spin particles is a formidable subject, where Lorentz-invariant approaches tend to lead to a rich gauge-symmetry structure, which serves to preserve the physical number of degrees of freedom. Introducing consistent interactions in such approaches is a non-trivial task, with most higher-spin Lagrangians specified only up to three points. In this talk, I will discuss a new, chiral description for massive higher-spin particles, which in four spacetime dimensions allows to do away with this kind of gauge symmetry. This greatly facilitates the introduction of consistent interactions. I will concentrate on three theories, in which higher-spin matter is coupled to electrodynamics, non-Abelian gauge theory or gravity. These theories are currently the only examples of consistent interacting field theories with massive higher-spin fields. The presented theories are chiral and have simple Lagrangians, resulting in Feynman rules analogous to those of massive scalars. In particular, I will discuss the resulting tree-level scattering amplitudes with two higher-spin matter particles and any number of positive-helicity photons, gluons or gravitons. These amplitudes were previously computed via on-shell recursion and provided evidence for the existence of such simple massive higher-spin theories.

We know that quantum gravity is expected to violate global symmetries of effective gravity theories. Black holes are expected to play a role in this violation. We discuss computations of gravity amplitudes, mainly involving scattering of spherically symmetric shells, that violate global symmetries. We will review prior work in two dimensions and we will discuss the new features that arise in higher dimensions.

In this talk, I will discuss the possibility that our universe lies near the boundary of the field space in string theory, including the theoretical challenges and the exciting phenomenological implications. These boundaries share some universal properties imposed by quantum gravity (sometimes promoted to Swampland constraints) that resemble our universe, like weak couplings, approximate global symmetries or small (time-dependent) vacuum energy. However, it remains as an open challenge to get an accelerated cosmology. We study whether the runaway behaviour of stringy scalar potentials towards in finite distance can produce an accelerated expanding cosmology a la quintessence, finding some potential examples in F-theory flux compactifications. I will discuss the caveats of these examples and the comparison to Swampland bounds. Furthermore, a universal feature of these regions is that there is a light infinite tower of states which is correlated to the value of the vacuum energy. I will show how experimental constraints force this tower to correspond to a KK tower (of mass of order neutrino scale) of a single extra mesoscopic dimension of order 10^{-6) m, which we denote as the Dark Dimension.